Method of consolidating a powder

ABSTRACT

A method of consolidating a powder ( 10 ) comprises the steps of filling an electrically conductive container ( 12 ) with the powder ( 10 ). Any air is evacuated out of the filled container ( 12 ), which is then sealed. The sealed container ( 12 ) is placed in a die ( 20 ) and a force is applied to consolidate the powder ( 10 ). Simultaneously an electric pulse and an ultrasonic pulse are applied to the container ( 12 ) as the powder ( 10 ) is consolidated. The electric and ultrasound pulses are applied during consolidation to disrupt the grain boundaries and assist in the fragmentation of any oxides.  
     The container ( 12 ) is then removed from the die ( 20 ) and from the consolidated powder ( 10 ).

The present invention relates to a method of consolidating a powder toproduce a component, join components or coat a component. In particularit relates to a method of consolidating a powder to produce newcomponents, join components or coat components suitable for aerospaceapplications.

Aerospace components require the use of high strength, high temperatureresistant alloys, which are notoriously difficult to process. As it isnot possible to weld or use other fabrication techniques on thesealloys, components are machined from billets. Machining the componentsfrom billets is time consuming, expensive and wasteful.

Powder metallurgy has been used to produce billets of these highperformance alloys from which components having complex geometries aremachined. Current powder processing routes for these alloys requireexpensive and wasteful processes, such as extrusion, to eliminate tracesof prior particle grain boundaries and produce low specificationcomponents.

The present invention seeks to provide a powder processing route whichovercomes the problems of prior particle grain boundaries and provides alow cost manufacturing route for components from these highspecification alloys.

According to one aspect of the present invention a method ofconsolidating a powder comprises the steps of filling an electricallyconductive container with powder, evacuating air out of the filledcontainer and sealing the filled container after evacuation, placing thesealed container in a die and applying a force sufficient to consolidatethe powder whilst simultaneously applying a electric pulse and anultrasound pulse thereto, removing the container from the die andremoving the container from the consolidated powder.

According to a second aspect of the present invention a method ofconsolidating a powder to join preforms comprising the steps of placingat least two preforms in abutting relationship in an electricallyconductive container, coating the abutting surfaces of the preforms withpowder, evacuating any air out of the container and sealing thecontainer after evacuation, placing the sealed container in a die andapplying a force sufficient to consolidate the powder and join thepreforms whilst simultaneously applying an electric pulse and anultrasound pulse to the container, removing the container from the dieand removing the container from the joined preforms.

According to a third aspect of the present invention a method ofconsolidating a powder comprises the steps of placing a preform in anelectrically conductive container, coating the surfaces of the preformwith powder, evacuating any air out of the container and sealing thecontainer after evacuation, placing the sealed container in a die andapplying a force sufficient to consolidate a coating of the powder ontothe preform whilst simultaneously applying an electric pulse and anultrasound pulse to the container, removing the container from the dieand removing the container from the coated preform.

The powder coating may have a different composition to the preforms andthe composition of each of the preforms may be different.

The electric and ultrasound pulses are applied during consolidation todisrupt the grain boundaries and assist in the fragmentation of anyoxides.

Preferably a high amplitude, high frequency electrical pulse is appliedto the container. The electrical pulse heats the surface of the powder,increasing the plasticity at the surface. Electrical energy in the rangeof 1-20 KHz is applied with a frequency of the order of 20 KHz.

The force used to compress the powder may be a mechanically inducedshock wave in the range of 5-20 GPa. The shock wave assists in thedisruption of the grain boundaries and helps destroy any oxides.

The ultrasound pulse, is of the order of 20 KHz, and is appliedsimultaneously with the shock wave to further disrupt the grainboundaries and to assist in the fragmentation of oxides.

Preferably the container is vibrated as it is filled with the powder.The powder may be a nickel alloy and the container may be made fromnickel, mild steel or stainless steel.

The consolidated powder may then be sintered or hot isostaticallypressed.

The present invention will now be described with reference to theaccompanying drawings in which;

FIG. 1 shows apparatus suitable for consolidating a powder in accordancewith present invention.

FIG. 2 shows apparatus suitable for joining preforms of consolidatedpowder.

FIG. 3 shows apparatus suitable for consolidating a powder coating ontoa preform.

Referring to FIG. 1, a nickel alloy powder 10 is encapsulated in acontainer 12. The container 12 is made from a ductile material, which iselectrically conductive and which will not contaminate the powder bydiffusion. In the preferred embodiment of the present invention thecontainer 12 is made from pure nickel, mild steel or stainless steelsheet. Electrically insulting connectors 14 are provided on either endof the container 12.

The container 12 is vibrated to pack the powder 10 down. A vacuum pump(not shown) is attached to a tube 16 on the container 12 and is used toevacuate the gas atmosphere surrounding the powder 10. Once the gas hasbeen evacuated from the container 12 the tube 16 is crimped and weldedshut.

The sealed container 12 is then placed into a die 20 having twoelectrically insulated connectors 22. The electrical connecters 22 areattached to a source of electrical energy, such as a capacitor bank (notshown).

The die 20 is closed and motor-driven hydraulic actuators (not shown)apply a force in the direction of arrows A to the die 20. A hydrostaticmedium 18, such as a fluid or elastomer, produces a shock wave that istransmitted to the powder filled container 10. The shock wave applies aforce, in the range of 5-20 GPa. The force necessary will depend uponthe type of powder and the size of the component. For a nickel alloypowder a shock wave of the order of 10 GPa is applied for a few tenthsof a microsecond to effect full consolidation.

As the force is applied to the die 20 the capacitor bank simultaneouslydelivers a high amplitude, high frequency pulse of electrical energy tothe connectors 22 on the die 20. For a nickel alloy powder a 1-20 KJpulse of electrical energy is delivered at a frequency of approximately20 KHz and an amplitude as high as the frequency switch system willallow. The electrical energy is transmitted to the connectors 14 on thecontainer 12. The electric energy is transmitted through the powder 10for of the order of 10 milliseconds. The electrical pulse has a waveformand amplitude that are tailored to disrupt grain boundaries and oxides.The electrical pulse is applied for of the order of 10 milliseconds suchthat it heats the surface of the powder 10 to increase the plasticitybut does not allow substantial heat conduction into the powder 10, whichcould cause micro structural alteration.

An ultrasound pulse of the order of 20 kHz is also superimposed onto theshock wave to further disrupt the grain boundaries and to assist in thefragmentation of any oxides.

Once the powder 10 has been consolidated the container 12 is removedfrom the die 20. The preform is then removed from the container 12,either by machining or by electrolysis. Boron nitride could be used as arelease agent to assist in the removal of the preform from thecontainer.

It is possible to fabricate complex components through the repeated useof the process. In FIG. 2 preforms 10 a, 10 b and 10 c, are placed inthe container 12. Mechanical compaction is aided by coating the abuttingsurfaces 11 of the preforms 10 a, 10 b, and 10 c with powder 10. Thepowder coated onto abutting faces 11 of the preforms 10 a, 10 b and 10 cmay be of a different composition. This is particularly beneficial whenthe preforms 10 a, 10 b and 10 c are formed from powders of dissimilarmaterials, which cannot be joined by conventional techniques such aswelding.

The container 12 is evacuated, sealed and placed into the die 20. Ashock wave is generated in the hydrostatic medium 18 whilst anelectrical pulse and ultrasound pulse is applied simultaneously to thecontainer 12 to join the preforms 10 a, 10 b and 10 c together.

The ultrasound pulse has a frequency of the order of 20 kHz and issuperimposed onto the shock wave to further disrupt the grain boundariesand to assist in the fragmentation of any oxides.

The container 12 is then removed from the die 20 and from the joinedpreforms.

The method can also be used to apply a powder coating to a preform.Referring to FIG. 3 a preform 10 d is placed in the container 12. Powder10 is placed around the surfaces of the perform 10 d. The powder coating10 may be of a different composition to the perform 10 d.

The container 12 is evacuated, sealed and placed into the die 20. Ashock wave is generated in the hydrostatic medium 18 whilst anelectrical pulse and ultrasound pulse is applied simultaneously to thecontainer 12.

Once the powder 10 has been consolidated the container 12 is thenremoved from the die 20 and from the coated preform 10 d.

The process described simultaneously compacts and disrupts grainboundaries and oxides in the powder 10. As the heat is not conductedinto the powder 10 this comparatively cold processing route allowsfine-grained preforms of consolidated powder to be produced. A finegrain structure is required to produce tough, fatigue resistantcomponents. The process thus enables the use of low cost manufacturingroute to produce high specification preforms of material from powder,join dissimilar performs together or apply powder coatings to thepreforms.

On completion of the process the consolidated preforms may be furtherprocessed depending on the material properties required for a particularapplication. For example the preforms of consolidated powder may besubsequently sintered or hot isotropically pressed.

1. A method of consolidating a powder comprising the steps of filling anelectrically conductive container with powder, evacuating any air out ofthe filled container and sealing the filled container after evacuation,placing the sealed container in a die and applying a force sufficient toconsolidate the powder whilst simultaneously applying an electric pulseand an ultrasonic pulse to the container, removing the container fromthe die and removing the container from the consolidated powder.
 2. Amethod as claimed in claim 1 in which electrical energy in the range of1-20 KJ is applied to the container.
 3. A method as claimed in claim 1in which the frequency of the electrical energy is of the order of 20KHz.
 4. A method claimed in claim 1 in which the force used to compressthe powder is a mechanically induced shock wave.
 5. A method as claimedin claim 1 in which the shock wave applies a force in the range of 5-20GPa.
 6. A method as claimed in claim 1 in which the ultrasound pulse hasa frequency of 20 KHz.
 7. A method as claimed in claim 1 in which thecontainer is vibrated as it is filled with powder.
 8. A method asclaimed in claim 1 in which the powder is a nickel alloy.
 9. A method asclaimed in claim 1 in which the container is made from nickel.
 10. Amethod as claimed in claim 1 in which the consolidated powder issintered after removal of the container.
 11. A method as claimed inclaim 1 in which the consolidated powder is hot isostatically pressedafter removal of the container.
 12. A method of consolidating a powderto join preforms comprising the steps of placing at least two preformsin abutting relationship in an electrically conductive container,coating the abutting surfaces of the preforms with powder, evacuatingany air out of the container and sealing the container after evacuation,placing the sealed container in a die and applying a force sufficient toconsolidate the powder and join the preforms whilst simultaneouslyapplying an electric pulse and an ultrasonic pulse to the container,removing the container from the die and removing the container from thejoined preforms.
 13. A method as claimed in claim 12 in which thecomposition of each of the preforms is different.
 14. A method asclaimed in claim 12 in which the powder coating has a differentcomposition to the preforms.
 15. A method as claimed in claim 12 inwhich electrical energy in the range of 1-20 KJ is applied to thecontainer.
 16. A method as claimed in claim 12 in which the frequency ofthe electrical energy is of the order of 20 KHz.
 17. A method claimed inclaim 12 in which the force used to compress the powder is amechanically induced shock wave.
 18. A method as claimed in claim 12 inwhich the shock wave applies a force in the range of 5-20 GPa.
 19. Amethod as claimed in any of claim 12 in which the ultrasound pulse has afrequency of 20 KHz.
 20. A method of consolidating a powder to coat aperform comprising the steps of placing at least one perform in anelectrically conductive container, coating the surfaces of the performwith powder, evacuating any air out of the container and sealing thecontainer after evacuation, placing the sealed container in a die andapplying a force sufficient to consolidate the powder into a coating onthe preform whilst simultaneously applying an electric pulse and anultrasonic pulse to the container, removing the container from the dieand removing the container from the coated preform.
 21. A method asclaimed in claim 20 in which the powder coating has a differentcomposition to the preforms.
 22. A method as claimed in claim 20 inwhich electrical energy in the range of 1-20 KJ is applied to thecontainer.
 23. A method as claimed in claim 20 in which the frequency ofthe electrical energy is of the order of 20KHz.
 24. A method claimed inclaim 20 in which the force used to compress the powder is amechanically induced shock wave.
 25. A method as claimed in claim 20 inwhich the shock wave applies a force in the range of 5-20 GPa.
 26. Amethod as claimed in claim 20 in which the ultrasound pulse has afrequency of 20 KHz.